with open('bill.txt','r+',encoding='utf-8') as f,open('bill.txt.bak','w',encoding='utf-8') as f_bak:
    #备份
    res = f.read()
    f_bak.write(res)
    #内容删除
    s = ""
    for line in res.split('\n'):
        #如果找不到返回-1
        if line.find('测试') ==-1:
            s += line + '\n'
    #回转指针
    f.seek(0)
    #清空内容
    f.truncate()
    #写入新内容
    f.write(s)